Metal surfaces are often prone to corrosion particularly when they are exposed to moisture in air, salt and acid/alkaline conditions. Since the process of corrosion is a gradual phenomenon, it is seldom noticed by the user before it produces sufficient damage. Metal panels commonly used in the automobile industry at different parts of the world are usually subjected to corrosion due to their exposure to adverse environments such as humidity, saline conditions at coastal regions, extremely hot and cold weather conditions. However, there are several ways of controlling corrosion. The selection of an anti-corrosive mechanism depends on safety conditions, economics and technical considerations. The most commonly used method of controlling corrosion to metal surfaces is by means of protective coatings in the form of paint or a coating material.
A wide range of coating materials is known in the art. For instance U.S. Pat. No. 3,847,651 discloses a process for providing an opaque waterproof protective coating film on a substrate which comprises a non-polar polymer dissolved in a non-aqueous solvent. U.S. Pat. No. 5,491,031 discloses a coating composition for metal substrates wherein the composition is free of a halide-containing vinyl polymer and comprises an epoxy novolac resin, a phenolic resin, polyester and an elastomer in a nonaqueous carrier. U.S. Pat. No. 6,191,056 discloses metallized, particularly aluminized, fabrics which are coated with specific polyurethane finishes and primer coatings comprising novel phosphate containing primers. EP 1712300 A discloses a method for coating metal surfaces with corrosion inhibiting polymer layers. US 2007/0014924 discloses a method for coating metal surfaces with corrosion inhibiting polymer layers. The composition disclosed therein comprises an organophosphorus compound comprising phosphorus and an alkyl group capable of interacting with a plastic monomer resin.
US 2007/0152192 discloses a zinc-rich preconstruction primer coating composition which includes a zinc powder dispersed in a polyurethane vehicle to which is added a curing catalyst to produce a moisture cured extended durability coating that is weldable.
Some of the already existing coating compositions have a zinc component which is widely used for its anticorrosive property. However, zinc containing compositions are not favoured in automobile industry due to the reason that zinc is a heavy metal and considered to be toxic at certain concentrations. Zinc chromate is one of the traditional anticorrosive pigments most frequently used in the formulation of primers. However, its environmental aggressiveness, toxicity and carcinogenic properties severely restrict its use and several alternatives were proposed to replace zinc chromate, zinc phosphate being one of them. Further, it has been reported that zinc phosphates may retard the growth of aquatic organisms and, therefore, is also becoming a target of ecological concern. Zinc phosphate and zinc oxide are designated as ecologically harmful substances in some European countries for this reason. Due to the growing awareness to the environmental harm associated with heavy metals, the market has been seeking non-toxic and cost effective replacements.
Further, although most of the coating materials exhibit the basic requirements like anticorrosion property, there is always a shortfall of OEM [Original equipment manufacturer] requirements such as drying, stickiness, and sagging. Also, during application, these products demand additional stabilization equipment to control and maintain sprayability, anti-settling and sagging requirements. In addition use of a zinc/epoxy based systems involves a complicated processes in application and use which makes it commercially not viable.
A need exists, therefore, for non-toxic anticorrosive pigment compositions free from harmful heavy metals and zinc, which will eliminate or ameliorate disadvantages associated with known non toxic pigment compositions and also completely fulfill all requirements regarding efficacy of anti-corrosive activity, processability and environmental safety.